1. Field of the Invention
The present invention relates to a membrane treatment method and apparatus for filtering a raw liquid such as sludge in an aeration tank for biological treatment, sludge discharged from the aeration tank, concentrated sludge obtained therefrom, waste water containing human excrement before being subjected to biological treatment, etc. More particularly, the present invention relates to a membrane treatment method and apparatus which can reduce costs, increase flux (flow volume per unit area of membrane), and decrease installation space.
2. Description of the Related Art
Waste water containing organic substances, nitrogen, phosphorus, etc., which would contaminate oceans, rivers and the like is generally subjected to biological treatment for conversion to clean water and is then discharged into a river, for example.
As means for separating the solid and liquid components of a reaction mixture resulting from biological treatment, a gravity-type settling tank has conventionally been used.
However, in recent years, a membrane separation technique has been used so as to reduce installation space and facilitate maintenance.
In such a membrane separation technique, steady production of filtrate over a prolonged period of time is very important. However, the volume of filtrate unavoidably decreases with passage of time. This problem is considered to be partly attributed to separated concentrated substances which deposit on the surface of a membrane and form a gel layer, which grows and hinders the passage of liquid to be filtered. The thickness of the gel layer increases as the concentration of contaminants in sludge increases and as the volume of filtrate increases. Accordingly, in the membrane separation technique, reduction in the thickness of the gel layer and removal of the generated gel layer are quite important.
Conventionally, a membrane treatment apparatus as shown in FIG. 5 is known. In FIG. 5, numeral 10 denotes a membrane apparatus, numeral 11 denotes a raw liquid tank for storing a raw liquid such as sludge, and numeral 12 denotes a pressurization pump. Numeral 13 denotes a frame which can be disassembled after removal of unillustrated packing seals. A plurality of membrane plates 14 are removably disposed within the frame 13. Each membrane plate 14 consists of a membrane support member 17 and membranes 18.
The membranes 18 are attached to both faces of the membrane support member 17 with a clearance 17a on each side.
Openings 15 and 16 for forming fluid passages are formed at upper and lower ends of each membrane plate 14, respectively.
Numeral 19 denotes discharge ports through which filtrate is discharged. Numeral 20 denotes a raw liquid inlet formed in the frame, and numeral 21 denotes a concentrated liquid outlet. Numeral 22 denotes inter-membrane passages through which raw liquid and/or concentrated liquid flows.
The raw liquid in the raw liquid tank 11 is led to the raw liquid inlet 20 by the pressurization pump 12. The raw liquid led to the membrane apparatus 10 flows into the inter-membrane passages 22 directly or via the opening(s) 16, so that the raw liquid is separated into concentrated liquid and filtrate that passes through the membranes 18.
The filtrate is led to the outside of the membrane apparatus 10 through the discharge ports 19. The concentrated liquid is returned to the raw liquid tank 11 via the concentrated liquid outlet 21 and is mixed with the raw liquid within the raw liquid tank 11. The above-described circulation is repeated by the action of the pressurization pump 12.
In general, the volume of liquid circulating within the inter-membrane passages 22 is determined on the basis of the flow rate of the liquid flowing through the inter-membrane passages 22. But, it is more important that the circulation volume is restricted depending on the diameter of the openings 15 and 16 formed at the upper and lower ends of the membrane plate 14. The openings 15 and 16 are designed to have a relatively large diameter such that a high circulation volume is secured in order to obtain a desired volume of filtrate; e.g., to have a diameter of about 65 mm.
Therefore, in order to conform to the relatively large openings, feed piping from the raw liquid tank 11 to the raw liquid inlet 20 and return piping from the concentrated liquid outlet 21 to the raw liquid tank 11 are designed to have a large diameter, thus increasing facility cost. Further, in addition to the piping, various types of accessories provided in the piping become larger, resulting in further increased facility costs.
In the conventional pressurized-type membrane processing apparatus using the pressurization pump 12, the horsepower (electrical power) of the pressurization pump 12 must be increased, since the volume of raw liquid fed from the raw liquid tank 11 to the inter-membrane passages 22 is large, and the raw liquid must be pressurized within the inter-membrane passages 22. Therefore, the conventional apparatus involves a problem of increased operating cost. Further, a pump of a large horsepower requires a large installation area.
When the membrane processing apparatus is operated in a state in which a pressure is applied to the raw liquid on the side of the membrane facing the inter-membrane passages 22 (on the side where sludge is circulated), the flow volume of filtrate increases temporarily. However, due to the increase in the flow volume of filtrate, growth of the gel layer on the membrane surface accelerates, with the result that the volume of filtrate decreases. In order to maintain a large flow volume of filtrate, higher power cost becomes necessary.
In order to solve the problems involved in the conventional pressurized-type membrane processing apparatus, a bubble-circulation-type membrane processing apparatus has been proposed.
As shown in FIG. 6, the proposed bubble-circulation-type membrane processing apparatus differs greatly from the conventional pressurized-type membrane processing apparatus in that no pressurization pump is used.
In FIG. 6, reference numeral 30 denotes a circulation tank disposed parallel to a membrane apparatus 10. The circulation tank 30 and the membrane apparatus 10 are connected with each other via an inlet pipe 32 for leading concentrated raw liquid to the membrane apparatus 10 and a discharge pipe 33 for discharging the concentrated raw liquid from the membrane apparatus 10. Thus, a circulation system is formed.
Reference numeral 40 denotes an aeration pipe inserted into the lower openings 16 and adapted to discharge fine air bubbles, reference numeral 41 denotes bubble discharge holes formed in the aeration pipe 40, and reference numeral 50 denotes a suction pump for suctioning filtrate.
The circulation tank 30 is constructed such that raw liquid is fed from an unillustrated raw liquid tank to a raw-liquid receiving port 31a, while excess concentrated liquid is allowed to overflow via a concentrated liquid discharge port 31b to thereby return to the raw liquid tank.
When air is supplied to the aeration pipe 40 to discharge fine bubbles from the discharge holes 41, within in the inter-membrane passages 22 there arises a difference in density between the raw liquid containing bubbles and the raw liquid newly supplied from the circulation tank 30. Due to this difference in density, a circulation flow is created between the membrane apparatus 10 and the circulation tank 30.
Meanwhile, filtrate is taken out to the outside via the discharge ports 19 by the action of the suction pump 50.
This apparatus offers the following advantage. Growth of gel layers on membrane surfaces is prevented, so that blocking due to sludge can be avoided while a large flow volume of filtrate is maintained. Further, sludge blocking can be prevented uniformly over the entire surface of membranes. Moreover, disassembly of the frame and cleaning of the membranes can be performed less frequently, and a pressurization pump of a large power can be eliminated, thereby contributing to a great reduction in cost.
The bubble-circulation-type membrane treatment apparatus can decrease cost as compared to the above-described pressurized-type membrane treatment apparatus. However, since the circulation flow is created by means of discharge of fine bubbles, the flow volume of liquid flowing through the inter-membrane passages is very small, resulting in a disadvantage of a low flux (rate of filtration per unit area of membrane).
Further, in recent years, there has been demand for reduction in space occupied by a membrane treatment apparatus that can treat a large volume of raw liquid. Therefore, development of a membrane treatment apparatus that meets such a requirement has been hoped for.